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A220896 Number of closed lambda-terms of size n with at most 2 free de Bruijn indices. 2

%I

%S 2,7,41,312,2784,27917,307943,3690055,47635777,658405747,9695617821,

%T 151488900012,2502346785164,43560247035581,796828655891895,

%U 15277217956632472,306268455537216444,6406216616616309707,139535240869213561821,3159084189417388736206

%N Number of closed lambda-terms of size n with at most 2 free de Bruijn indices.

%H Katarzyna Grygiel and Pierre Lescanne, <a href="http://arxiv.org/abs/1210.2610">Counting and generating lambda-terms</a>, arXiv preprint arXiv:1210.2610, 2012

%F Grygiel and Lescanne give a recurrence.

%t T[0, m_] := m; T[n_, m_] := T[n, m] = T[n-1, m+1] + Sum[T[i, m] T[n-i-1, m], {i, 0, n-1}];

%t a[n_] := T[n, 2];

%t Table[a[n], {n, 0, 14}] (* _Jean-Fran├žois Alcover_, May 23 2017 *)

%K nonn

%O 0,1

%A _N. J. A. Sloane_, Dec 31 2012

%E More terms from _Alois P. Heinz_, May 23 2017

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Last modified October 25 19:43 EDT 2021. Contains 348255 sequences. (Running on oeis4.)